A new perspective on the composition-structure-property relationships on Nb/Mo/Cr-doped O3-type layered oxide as cathode materials for sodium-ion batteries

The layered transition metal oxides are an essential class of cathode materials in sodium-ion batteries. However, the capacity decay mechanisms of many materials at a wide voltage window are still ambiguous. Therefore, research on the intrinsic structure of materials at the electronic or atomic level is of great significance to unlock the potential of more materials. In this study, a new family of layered NaNi0.45Mn0.3Ti0.2M0.05O2 (M = Nb/Mo/Cr) has been demonstrated to combine high specific capacity (NaNi0.45Mn0.3Ti0.2Cr0.05O2, 12 mA g−1, 185.7 mAh g−1, 2-4.2 V) with preferable Na+ insertion/extraction activity (1920 mA g−1, 111.2 mAh g−1). The participation of Ni2+/Ni3+ and Cr3+/Cr4+/Cr6+ in the redox reaction upon cycling significantly contributes to the superior specific capacity, and the diffusion kinetics calculation indicates that the substitution of Cr for Ni can create a more suitable path with a low energy barrier for Na+ movement, which is particularly crucial for the rate performance. Besides, the unfavorable cycling stability is affected by the distortion of the crystal structure, suggesting that the Jahn-Teller effect of Mn3+/Ni3+ and the second-order Jahn-Teller effect of Nb5+/Mo6+ (d0 transition metal) together with Cr6+ migration caused by disproportionation of Cr4+ have inescapable responsibilities. Additionally, the reversible phase transition between the O3 and P3 phases upon Na+ insertion/extraction at a wide voltage window is identified by in situ XRD measurements and verified to be the key factor of the high coulombic efficiency for NMTCr. An in-depth understanding of NMTNb and NMTMo can provide new insights for material design and exploitation. The composition-structure-property relationships behind the electrochemical performance can be comprehended profoundly by coupling the experimental verification with density functional theory calculations.